Digitalized speed sensitive moving strand detection apparatus

ABSTRACT

Apparatus is provided for detecting the oscillation of a moving strand of yarn at a particular frequency as it is being wound about a bobbin. The strand when oscillating, impinges more than once upon photoelectric detector cells moved past a detection station to thereby give electric output pulses as an indication of proper yarn feed. Indicator signals may be provided by counting the cell output pulses or by measuring the time between successive pulses. Multiple cells may be presented in sequence with two signal processing channels being used to provide higher reliability or detection speeds and to eliminate spurious signals produced by either a-c or d-c noise superimposed upon the photocells.

United States Patent [1 1 Erbstein [11] 3,772,524 ['4 Nov. 13, 1973 DIGITALIZED SPEED SENSITIVE MOVING STRAND DETECTION APPARATUS Robert Stephen Erbstein, Coventry, R.I.

Leesona Corporation, Warwick, RI.

Jan. 3, 1972 [75] Inventor:

Assignee:

Filed:

Appl. No.:

3,672,143 6/1972 Whitney 57/81 X it 1 l 20 24 Primary Examiner-James W. Lawrence Assistant Examiner-T. N. Grigsby AttorneyAlbert P. Davis et al.

[57] ABSTRACT Apparatus is provided for detecting the oscillation of a moving strand of yarn at a particular frequency as it is being wound about a bobbin. The strand when oscillating, impinges more than once upon photoelectric detector cells moved past a detection station to thereby give electric output pulses as an indication of proper yarn feed.

Indicator signals may be provided by counting the cell output pulses or by measuring the time between successive pulses. Multiple cells may be presented in sequence with two signal processing channels being used to provide higher reliability or detection speeds and to eliminate spurious signals produced by either a-c or d-c noise superimposed upon the photocells.

9 Claims, 8 Drawing Figures PAIENIEUHUV 131975 SHEET 10F 2 FIG?) DIGITALIZED SPEED SENSITIVE MOVING STRAND DETECTION APPARATUS This invention relates to apparatus for detecting presence of a moving strand such as yarn being wound upon a bobbin, and more particularly it relates to apparatus sensing the difference between stationary and moving strands.

In textile machinery it is common to wind strands upon a bobbin. Thus, several strands of a roving may be twisted together to provide a strand of yarn which is wound upon a bobbin by a mechanism which rotates the yarn about said bobbin and thus, may further twist it while winding it thereon. The textile machinery may malfunction and the strand may become broken, in which condition the broken end of the strand may hang down without being wound upon the bobbin. Also, the bobbin winding mechanism may become jammed or the apparatus may otherwise operate improperly so that the strand is wound on the bobbin at less than normal speed. It is therefore desirable to detect the presence of a moving strand that is being wound at normal speed upon the bobbin as an indication of proper machine operation.

While moving strand detectors have been provided in the prior art they have been deficient in certain respects. For example, they may give false signals from oscillation of fluorescent ambient lighting or by noise signals of various sorts, either produced by electrical interference or ambient light interference. Furthermore, these prior art detectors are not generally speed sensitive and, therefore, they cannot tell when the strand is moving at the proper speed.

It is therefore a general objective of the invention to provide improved movable strand detection apparatus.

A more'speciflc objective of the invention is to provide improved apparatus to differentiate between a stationary and a moving strand. A further object of the invention is to provide apparatus to distinguish the movement speed of a strand.

Other features, advantages and objectives of the invention will be found throughout the following detailed description made with reference to the accompanying drawing, wherein:

FIG. 1 is a partial schematic sketch of a fault detection system operable in accordance with this invention for indicating proper yarn feed in textile machinery,

FIG. 2 is a schematic array of a set of photoelectric detectors arranged in accordance with the teachings of this invention, to detect moving strands,

FIG. 3 is a schematic block circuit diagram of a mov ing strand detector embodiment afforded by the invention,

FIG. 4 is a waveform diagram chart showing waveforms A to 1 associated with the operation of the detector embodiment of FIG. 3,

FIG. 5 is a schematic block diagram of an improved portion of the detector of FIG. 3,

FIG. 6 is a block diagram of a further detector embodiment afforded by the invention,

FIG. 7 is a block diagram of an alternative detector embodiment of the invention, and

FIG. 8 is a waveform diagram illustrating operation mechanism 14 rotating about the outside of the bobbin 13 in the direction 15 to thereby cause the strand 12 to oscillate back and forth between limits such as 12 and 12 which oscillation may be observed at an area 16, which may be termed a detection station. The frequency of oscillation of the strand 12 is predetermined by the normal winding speed of the mechanism 14. Textile machinery of this nature is well known in the art and the details other than shown do not significantly relate to an understanding of the present invention and might tend to obscure the nature of the invention. However, it is to be recognized that if the strand or yarn l2 breaks it will hang free without oscillation in a position such as phantom showing 12", and in certain other malfunction conditions the winding mechanism 14 might not be operating at proper speeds, so that the oscillation frequency could be slower than normal, for example.

A typical textile winding mechanism may have a plurality of bobbin stations 16, etc., alongside each other, so that each station may be observed in sequence by a service carriage 20 moving across the detection stations in the direction of arrow 21 to detect the condition of the station and automatically load and connect bobbins. Equipment of this nature is described in U. S. Pat. No. 3,403,866, C. C. Bell et al., Oct. 1, 1968, for example.

In accordance with the present invention, two or more photoelectric cells (PEC) 22, 23 are spaced apart with a gap 24 between them in the carriage so that they are moved across detection stations 16. They may be provided with appropriate light sources, slits, and lens systems (not shown) so that the strand 12 is focused upon an active plane of the photo cells 22, 23 for detection of the strand as it impinges upon the edges of the cells while moving across the detection stations 16.

With the gap 24 between the two cells 22, 23, in position at station 16, the strand 12 will tend to oscillate back and forth and impinge upon each of the cells 22, 23 a plurality of times giving more than one output pulse per cell unless the speed of the carriage is too fast or the oscillation frequency of the strand 12 is too slow. However, the broken, hanging strand 12" will only impinge upon each cell once and, thus, can give only one output pulse per cell. Thus, we may assume that only the leading edge of each photo cell signal is used and this will identify the impingement of the strand 12 on one of the edges of the cell. Accordingly, by providing means for distinguishing only those cases where a cell gives more than one pulse per strand across the detection station 16, a distinction can be made between broken stationary strands and moving strands. Likewise, when the speed of carriage movement, gap spacing 24, and strand oscillation frequencies are taken into account, a normal system will provide a predetermined number of pulses as the edge of a photo cell or the ga 24 crosses the detection station.

In the event the speed of the carriage is high, or the strand oscillation frequency is low, an array of several cells providing a plurality of gaps 24 may be arranged as shown in FIG. 2, where the operation is basically the same except for a 'redundance which will increase detector reliability and a plurality of gaps that should reduce the possibility of missing oscillations at a single gap. It might be possible to use a single cell with light barrier gaps, but as shown in the embodiment of FIGS. 1 and 2, separate cells A and B are used so that corresponding terminals A and B are connected to alternate cells 22, 23 presented in sequence by the scanning carriage 20. These signals are then processed in two separate channels as shown in the block circuit diagram of FIG. 3.

The reliability may be further enhanced by altering the cell-gap geometry. Consider the photo cell gap spacing A B where the effective scanning distance for detecting two pulses from an oscillating strand is small as compared withthe scanning distance A B which would indicate a hanging or stationary strand. Further decreasing of A B while enlarging or keeping A B the same will increase the possibility of differentiation between the hanging and moving strands based on the non sequential nature of the moving strand as compared to the hanging, i.e., the moving strand may hit cell 23 at B then return to A since A B is very small. This will either generate a higher count or a shorter tune interval corresponding to the. smaller distance.

With attention to FIG. 3, signals of cells 22 and 22 are presented at terminal 30. Capacitor 31 permits only a-c pulse signals to be transmitted to amplifier circuit 32, and thus, eliminates any d-c or slowly varying a-c noise signals that might be introduced by ambient light or electrical current levels, for example. Amplifier 32 increases the magnitude of the desired signal while further reducing low frequency noise and also diminishing high frequency noise. The channel for processing signals B from the cells 23 operates similarly.

Amplifiers 33 and 34 are threshold detectors, converting the analog inputs to digital signals, where the threshold level is set by potentiometer 35. Accordingly NAND 38 inhibits any signals occurring on channels A and B simultaneously such as might occur from line voltage surges or overhead ambients. Since the array geometry and optics are chosen so that a strand image, as viewed at the photo cells, is always smaller than the spacing between cells, the above condition does not effect a properly generated signal.

In essence the signal pulses are digitally handled in this circuit so that if equal pulses appear at leads 36 and 37 then NAND circuit 38 closes AND gates 39 and 40 and inhibits passage of any signals. However, if a pulse appears on only one line 36 or 37 then it passes the AND GATE 39 or 40 and operates a one-shot multivibrator 41 or 42 to provide an output pulse of a timed duration relates to the normally expected oscillation period of the strand to be detected. Thus, then if the strand impinges sequentially on cells 22 and 23 across gap 24, assume a pulse will appear first at input A and operate multivibrator 41. If then a pulse appears on input B and operates multivibrator 42 before the time period of multivibrator 41 expires, then AND gate 43 is operated to energize an indicator such as multivibrator 44 which will give a pulse indicating the strand is properly oscillating.

The waveforms of FIG. 4 will illustrate operation of the foregoing circuit and the respective waveforms are referenced to FIG. 3 circuit points by the letters A through I. Each waveform is shown on the same time scale starting at the left so that the vertical phantom lines will show occurrences at a particular time instant.

A first cell A may encounter the oscillating strand at a leading edge A and also at a trailing edge A each producing a signal pulse. Also, the next successive cell B may encounter the oscillating strand at the leading edge B once or twice depending upon strand oscillation frequency and phase, the first pulse encountered at times the phasing permits being shown in phantom at B Whenever the strand is stationary, then only the pulses A and B separated by Time T would be present. If the strand is oscillating at least two pulses separated by the shorter time T would be present. If no strand is viewed, then no pulses are present, therefore an indication of oscillation shows proper operation and, conversely, no oscillation shows some sort of improper operation such as a stationary strand or lack of a strand.

A d-c level would vary at the photo cell before capacitor 31 as shown in A and B waveforms with an AC wave 45 superimposed as might be derived from fluorescent lighting, for example, or other periodic interference and not appear at the output of amplifier 32 because of the filtering action of capacitors 31 and 31 Similarly a high frequency moise will be reduced by capacitor 31".

A noise of frequency and amplitude sufficient to exceed the threshold set by adjustment 35 noise be cancelled by action of the gating circuits hereinbefore described.

At the output of NAND circuit 38 appears waveform C, which inhibits noise pulses N and N but allows pulses A, B A, and B to pass.

Accordingly, pulse A is passed through AND gate 39 to lead D as is pulse A and conversely pulses B and B are passed through AND gate 40 to lead E.

Thus, multivibrator 41 is fired by pulses A and A as shown on waveform F. Likewise, multivibrator 42 is fired by pulse B as shown on waveform G, so that the output of AND gate 43 on waveform I-I occurs during the overlap time of pulses from multivibrators 41 and 42. If a particular length or shaped pulse output indication is required multivibrator 44 may convert the output to one of standard length as shown on waveform 1.

Accordingly, the circuit arrangement of FIG. 3 is advantageous in eliminating the possibility of erroneous signals and is operable with digital systems so that pulse signals may be used to control local or remote indicators or equipment should there be improperly operable equipment necessary. This apparatus may be used to detect whether vibrating strands are vibrating at proper speeds in any sort of equipment but is useful in particular with textile winding and reeling equipment. It is to be recognized that those skilled in the art may choose the variations of response times of the multivibrators to accept different periods of vibration of the strand and different transit times or scanning speeds of the carriage. In general the multivibrator period T should be slightly longer than the period of oscillation T but much shorter than the time period T between pulses produced by a stationary strand.

In FIG. 5 is shown a modified one-shot multivibrator circuit 50 which may be used to avoid missing a strand detection signal under circumstances that might be presented, for example, as the oscillating yarn passes the first cell. In all events, a positive going pulse 54 triggers the one-shot multivibrator 50 to produce an output pulse 52 having a specified duration period which will span two adjacent pulses produced by an oscillating strand. In this embodiment, the multivibrator 50 is reset whenever a second input pulse 54 is received while the output pulse 52 is present by means for exam ple, of the AND circuit 51. Thus both pulses B and B of FIG. 4 would provide an output signal.

A pulse shaper may be used, therefore, to assure an output pulse 57 of fixed duration even when multivibrator 50 is reset during the presence of an output pulse. Thus, a differentiator 53 may derive a pulse 55 from the leading edge of waveform 52 and produce in pulse stretcher 56 the pulse 57 of required duration for processing in AND circuit 43 as explained hereinbefore in connection with the embodiment of FIG. 3. Also, a similar pulse stretching circuit 58 with amplifier 59 may be used in the output circuit rather than a multivibrator.

As may be seen in FIG. 6, a counter 61 may give an output at a particular count on lead 63. Thus, pulses from cells A and B may be presented to OR circuit 60 for counting the counter. If the number of pulses exceed the number of cells, then an output is produced at lead 63. Thus, for two cells if the strand is stationary then only two pulses are produced. If the counter provides an output at lead 63 on a three count, then no output is obtained. However, an oscillating strand should produce at least one extra pulse at one of the cells so that a three count will be produced. Similarly, if three cells were used, a four count would indicate an oscillating strand. The counter is reset at lead 62 before each carriage starts its scan across the detection station.

If it is necessary to eliminate noise signals as in the two channel processing system of FIG. 3, then the signals A and B can be processed through gates 39 and 40 into the counter of FIG. 6.

The counter 61 can also be a binary counter providing an output at the two count when used with a single cell. Thus, an oscillating strand could produce a second pulse by oscillating across either the leading or trailing edge of the cell. If the oscillation frequency of the strand is high and several pulses per cell are normally expected, then a higher required count can signify that the oscillation frequency is proper.

As may be seen in the embodiment of FIG. 7, a single cell may be used in like manner to generate an output if two closely spaced pulses are produced by an oscillating strand impinging upon the leading or trailing edge of a single cell. Accordingly, multivibrator 70 produces an output pulse with a duration period T, slightly greater than the oscillation period of the strand.

As may be seen from FIG. 8, the input pulse A is delayed by delay block 71 before triggering multivibrator 70 so that the multivibrator output waveform 72 will not gate the input pulse at AND gate 73, therefore indicating the oscillating strand is missing. However, when the oscillating strand produces two pulses A and A- then the pulse A is gated by multivibrator waveform 72' and the output pulse 75 from the AND circuit 73 indicates the presence of an oscillating strand.

It is evident, therefore, that the oscillating strand may be detected with a counter or a time sensitive electronic circuit, and distinguished from a stationary strand or a missing strand, with either single or multiple cell blocks.

What is claimed is:

1. Apparatus for detecting a movable strand oscillating back and forth across a detection station by means of a movable detection carriage passing said detection station, comprising in combination, at least two detection means moving in said carriage past said station in sequence and sensing the strand in each detection means as it impinges upon it at either end of a sensing span distance crossing the detection station, separate ones of said detection means being separated from each other by a sensing gap distance, means providing for the speed of movement of said carriage and the oscillation frequency of said strand comprising a critical spacing of said sensing gap proportioned to produce a signal on two different adjacent detection means as the strand impinges thereon while oscillating across said gap at said station, and distinguishing means to sense the oscillation of said strand across the gap and provide an indication only when the strand oscillates across the gap to produce more than one signal on one of said detection means wherein the distinguishing means comprises a counter operable to count as the carriage passes the detection station and set to provide an output signal constituting said indication at a predetermined count of signals from all said detection means, passing said station, each count taken upon impingement of said strand upon any one of the detection 2. Apparatus for detecting a movablestrand oscillating back and forth across a detection station by means of a movable detection carriage passing said detection station, comprising in combination, at least two detection means moving in said carriage past said station in sequence and sensing the strand in each detection means as it impinges upon it at either end of a sensing span distance crossing the detection station, separate ones of said detection means being separated from each other by a sensing gap distance, means providing for the speed of movement of said carriage and the oscillation frequency of said strand comprising a critical spacing of said sensing gap proportioned to produce a signal on two different adjacent detection means as the strand impinges thereon while oscillating across said gap at said station, and distinguishing means to sense the oscillation of said strand across the gap and provide an indication only when the strand oscillates across the gap to produce more than one signal on one of said detection means including means responsive to signals from both said detection means to inhibit said indication when signals are simultaneously produced by both said detection means.

3. Apparatus for detecting a movable strand oscillating back and forth across a detection station by means of a movable detection carriage passing said detection station, comprising in combination, at least two detection means moving in said carriage past said station in sequence and sensing the strand in each detection means as it impinges upon it at either end of a sensing span distance crossing the detection station, separate ones of said detection means being separated from each other by a sensing gap distance, means providing for the speed of movement of said carriage and the oscillation frequency of said strand comprising a critical spacing of said sensing gap proportioned to produce a signal on two different adjacent detection means as the strand impinges thereon while oscillating across said gap at said station, and distinguishing means to sense the oscillation of said strand across the gap and provide an indication only when the strand oscillates across the gap to produce more than one signal on one of said detection means including a plurality of detection means presented at said detection station in sequence to thereby present a plurality of said sensing gaps and means combining outputs of alternative detection means to produce only two corresponding sensed signals therefrom.

4. Apparatus for detecting at a detection station, a movable strand which is normally at said station oscillating back and forth at a predetermined frequency by means of a movable detection carriage passing said detection station comprising in combination, a detector moved by said carriage past said station responsive to provide an electronic signal when said strand impinges upon said detector, and means moving said detector across said station at a speed proportional to the speed of said carriage and the frequency of the oscillation of the strand to cause the impingement of the oscillating strand more than once upon said detector as it moves past said station to provide more than one pulse output signal from said strand, and means providing from said pulse signals an indication of a condition whether the strand is oscillating comprising time discriminating means providing said indication when two said electronic signals are received within the oscillation period of said strand.

5. Apparatus for detecting at a detection station, a movable strand which is normally at said station oscillating back and forth at a predetermined frequency by means of a movable detection carriage passing said detection station comprising in combination, a detector moved by said carriage past said station responsive to provide an electronic signal when said strand impinges upon said detector, and means moving said detector across said station at a speed proportional to the speed of said carriage and the frequency of the oscillation of the strand to cause the impingement of the oscillating strand more than once upon said detector as it moves past said station to provide more than one pulse output signal from said strand, and means providing from said pulse signals an indication of a condition whether the strandis oscillating comprising counter apparatus connected to provide said indication when the strand impinges on the detector more than once at said station.

6. Apparatus for detecting a movable strand oscillating back and forth across a detection station by means of a movable detection carriage passing said detection station, comprising in combination, at least two detection means moving in said carriage past said station in sequence and sensing the strand in each detection means as it impinges upon it at either end of a sensing span distance crossing the detection station, separate ones of said detection means being separated from each other by a sensing gap distance, means providing for the speed of movement of said carriage and the oscillation frequency of said strand comprising a critical spacing of said sensing gap proportioned to produce a signal on two different adjacent detection means as the strand impinges thereon while oscillating across said gap at said station, and distinguishing means to sense the oscillation of said strand across the gap and provide an indication only when the strand oscillates across the gap to produce more than one signal on one of said detection means wherein said distinguishing means comprises time sensing means responsive to provide said indication when the time between detection of the strand at said two detection means is short enough to signify the oscillation of said strand across said sensing gap and wherein said time sensing means comprises a digital logic circuit arrangement responsive to and processing strand sensing signals from said two detection means in two signal processing channels, each having means providing an output signal of predetermined time duration for each strand sensing signal, and comparison means producing said indication when said signal of predetermined time duration is present in both signal processing channels simultaneously.

7. Sensing apparatus as defined in claim 6, including means responsive to signals in both said detection means for blocking signals in said channels when signals in said detection means occur simultaneously.

8. Apparatus as defined in claim 6, wherein said means providing an output signal of predetermined time duration comprise one-shot multivibrators.

9. Apparatus as defined in claim 8, wherein said multivibrators are of the type that will be reset if another signal occurs before the multivibrator time period expires, including a pulse shaping circuit in said channels for producing an output pulse of predetermined duration for each multivibrator output signal. 

1. Apparatus for detecting a movable strand oscillating back and forth across a detection station by means of a movable detection carriage passing said detection station, comprising in combination, at least two detection means moving in said carriage past said station in sequence and sensing the strand in each detection means as it impinges upon it at either end of a sensing span distance crossing the detection station, separate ones of said detection means being separated from each other by a sensing gap distance, means providing for the speed of movement of said carriage and the oscillation frequency of said strand comprising a critical spacing of said sensing gap proportioned to produce a signal on two different adjacent detection means as the strand impinges thereon while oscillating across said gap at said station, and distinguishing means to sense the oscillation of said strand across the gap and provide an indication only when the strand oscillates across the gap to produce more than one signal on one of said detection means wherein the distinguishing means comprises a counter operable to count as the carriage passes the detection station and set to provide an output signal constituting said indication at a predetermined count of signals from all said detection means, passing said station, each count taken upon impingement of said strand upon any one of the detection means, and said predetermined count being higher than the number of said detection means presented by said carriage at said detection station, whereby said strand must oscillate across said gap at said detection station to aChieve said predetermined count.
 2. Apparatus for detecting a movable strand oscillating back and forth across a detection station by means of a movable detection carriage passing said detection station, comprising in combination, at least two detection means moving in said carriage past said station in sequence and sensing the strand in each detection means as it impinges upon it at either end of a sensing span distance crossing the detection station, separate ones of said detection means being separated from each other by a sensing gap distance, means providing for the speed of movement of said carriage and the oscillation frequency of said strand comprising a critical spacing of said sensing gap proportioned to produce a signal on two different adjacent detection means as the strand impinges thereon while oscillating across said gap at said station, and distinguishing means to sense the oscillation of said strand across the gap and provide an indication only when the strand oscillates across the gap to produce more than one signal on one of said detection means including means responsive to signals from both said detection means to inhibit said indication when signals are simultaneously produced by both said detection means.
 3. Apparatus for detecting a movable strand oscillating back and forth across a detection station by means of a movable detection carriage passing said detection station, comprising in combination, at least two detection means moving in said carriage past said station in sequence and sensing the strand in each detection means as it impinges upon it at either end of a sensing span distance crossing the detection station, separate ones of said detection means being separated from each other by a sensing gap distance, means providing for the speed of movement of said carriage and the oscillation frequency of said strand comprising a critical spacing of said sensing gap proportioned to produce a signal on two different adjacent detection means as the strand impinges thereon while oscillating across said gap at said station, and distinguishing means to sense the oscillation of said strand across the gap and provide an indication only when the strand oscillates across the gap to produce more than one signal on one of said detection means including a plurality of detection means presented at said detection station in sequence to thereby present a plurality of said sensing gaps and means combining outputs of alternative detection means to produce only two corresponding sensed signals therefrom.
 4. Apparatus for detecting at a detection station, a movable strand which is normally at said station oscillating back and forth at a predetermined frequency by means of a movable detection carriage passing said detection station comprising in combination, a detector moved by said carriage past said station responsive to provide an electronic signal when said strand impinges upon said detector, and means moving said detector across said station at a speed proportional to the speed of said carriage and the frequency of the oscillation of the strand to cause the impingement of the oscillating strand more than once upon said detector as it moves past said station to provide more than one pulse output signal from said strand, and means providing from said pulse signals an indication of a condition whether the strand is oscillating comprising time discriminating means providing said indication when two said electronic signals are received within the oscillation period of said strand.
 5. Apparatus for detecting at a detection station, a movable strand which is normally at said station oscillating back and forth at a predetermined frequency by means of a movable detection carriage passing said detection station comprising in combination, a detector moved by said carriage past said station responsive to provide an electronic signal when said strand impinges upon said detector, and means moving said detector across said station at a speed proportional to the Speed of said carriage and the frequency of the oscillation of the strand to cause the impingement of the oscillating strand more than once upon said detector as it moves past said station to provide more than one pulse output signal from said strand, and means providing from said pulse signals an indication of a condition whether the strand is oscillating comprising counter apparatus connected to provide said indication when the strand impinges on the detector more than once at said station.
 6. Apparatus for detecting a movable strand oscillating back and forth across a detection station by means of a movable detection carriage passing said detection station, comprising in combination, at least two detection means moving in said carriage past said station in sequence and sensing the strand in each detection means as it impinges upon it at either end of a sensing span distance crossing the detection station, separate ones of said detection means being separated from each other by a sensing gap distance, means providing for the speed of movement of said carriage and the oscillation frequency of said strand comprising a critical spacing of said sensing gap proportioned to produce a signal on two different adjacent detection means as the strand impinges thereon while oscillating across said gap at said station, and distinguishing means to sense the oscillation of said strand across the gap and provide an indication only when the strand oscillates across the gap to produce more than one signal on one of said detection means wherein said distinguishing means comprises time sensing means responsive to provide said indication when the time between detection of the strand at said two detection means is short enough to signify the oscillation of said strand across said sensing gap and wherein said time sensing means comprises a digital logic circuit arrangement responsive to and processing strand sensing signals from said two detection means in two signal processing channels, each having means providing an output signal of predetermined time duration for each strand sensing signal, and comparison means producing said indication when said signal of predetermined time duration is present in both signal processing channels simultaneously.
 7. Sensing apparatus as defined in claim 6, including means responsive to signals in both said detection means for blocking signals in said channels when signals in said detection means occur simultaneously.
 8. Apparatus as defined in claim 6, wherein said means providing an output signal of predetermined time duration comprise one-shot multivibrators.
 9. Apparatus as defined in claim 8, wherein said multivibrators are of the type that will be reset if another signal occurs before the multivibrator time period expires, including a pulse shaping circuit in said channels for producing an output pulse of predetermined duration for each multivibrator output signal. 